Nobody else is concerned that she gets a disk space low warning and it goes away after reboot?
Do you guys ever try to solve problems? Or is this just a place to get people to spend money on new hardware? This has nothing to do with the amount of RAM in the computer, nor does it have anything to do with the cache size of a Celeron D processor. Also 40 gigs is plenty of hard drive space for someone that doesn't game, or store a multi thousand song collection of mp3s, or lots of movies.
Sounds like a program is writing a bunch of temp files to the disk, and those are getting deleted on reboot. If things are working correctly this doesn't happen, but if some sort of malware is involved that could explain this.
Another explanation could be ridicously large temp from web browsing.. but that seems unlikely.
I recommend a full scan from trend micro's house call:
http://housecall.trendmicro.com/ . Then running hijack this.
Check out the Security forum for help with hijackthis.